Phototube containing means to counteract negative wall charges



A. H. TAYLOR March 30, 1948. 2,438,587

PHOTOTUBE CONTAINING MEANS TO COUNTERACT NEGATIVE WALL CHARGES Filed Dec. 24, 1943 VaL T5 m a r m r .T O n w+ VJ M Patented Mar. 30, 1948 I PHOTOTUBE CONTAINING MEANS TO COUN- TERACT NEGATIVE WALL CHARGES Augustus Hadley Taylor, East Cleveland, Ohio,

assignor to General Electric Company, a corporation of New York Application December 24, 19 13, Serial No. 515,472

Claims. 1

This invention relates to electric discharge devices in which the discharge is a space current consisting of electrons, as distinguished from a current in ionized gas or vapor, and relates particularly to photoelectric cells and the like. The invention aims at minimizing the impedance offered by the cell to the passage of current under any given conditions of applied voltage and incident light, thus allowing the current to reach a desired value under a lower applied voltage on the cell. The invention is here explained with particular reference to a simple type of photocell haVing an envelope formed of a vitreous tube with sheet metal end walls or headers, and having a cathode surfaced or activated with an antimony and caesium combination; but it will be under stood that it is applicable to other types of photocells and to other'types of discharge devices, such as electron multipliers, for example. Various features and advantages of the invention will appear from the description of species and forms of embodiment, and from the drawings.

In the drawings, Fig. 1 shows averticallongitudinal section through a photocell conveniently embodying the invention; Fig. 2.is a tilted side view of a photocell embodying the invention in a different form; Fig, 3 is a side view of'a cell embodying the invention in another form; and Fig. 4 is a diagram showing current and voltage curves characteristic of the several cells shown in Figs. 1, 2, and 3,

A photocell device X is shown in the figure as comprising a short vitreous or glass envelope tube ill having its ends sealed by fusion to the margins of abutting annularly embossed discs or electrode headers l l, I?! of sheet metal (such as chrome iron or ,Allegheny 55, as it is commercially known), generally similar to those used for the ends of the tubular incandescent lamps commercially known as Lumiline lamps. The cathode header II which appears at the bottom in Fig. 1 carries a coaxial sheet metal cathode disc It (as of Nichrome or nickel) welded thereto and shown with its outer edge turnedup toward the anode header 92. In practice, the tube It] may be of about inch cylindrical bore and about inch long in the axial direction. The sealed-01f remainder of a glass appendix or exhaust tube 14 is shown attached or sealed by fusion to the outer side of the centrally apertured header H, substantially coaxial with its aperture and with a corresponding central opening lfi'in the disc 13. The anode header I2 which appears at the top of the figure is not apertured, but carries a metal part It (as of nickel foil) that presents a substantially fiat surface toward the cathode I3. Current connections I9, H? are shown welded to the headers ll, I2. The cathode 13 may have a light-sensitive photoelectric emitting surface of any suitable character, such, for example, as an antimonycaesium combination formed by thermal deposition on the cathode first of antimony and then of caesium. The cell X is evacuated so thoroughly that current through it due to Voltage across the electrodes is essentially a space current carried by electrons emitted from the cathode I3, as distinguished from a current in ionized gas or vapor.

As thus far described, the cell X and its mode of operation correspond essentially to those already known.

In the use of such a cell X, the current leads l9, E9 of the anode and cathode electrodes at opposite sides or ends of the cell (it is convenient to refer to the parts ll, I2 as ends, without regard to the relative dimensions of the cell vertically and horizontally in Fig, 1) are connected across a suitable constant voltage source V as shown in Fig. 1, and a variable ray of lighter other radiation R falls diagonally or slopingly on the cathode l3, after passing through the vitreous wall lit. The current through the cell under a volta e V of any given value varies according to or in proportion to the strength of the ray R. While the saturation current of the cell X is only reached under a relatively high voltage V, of the order of some 135 volts when the cell is constructed and proportioned as above described, per cent of this current is reached under a very much lower voltage, of the order of only a few volts. It is desirable that some 90 per cent of the saturation should be reached under a relatively low voltage, lower, indeed, than is generally the case with the cells X heretofore known.

In accordance with this invention, the cell X is provided with a conductor or conductors extending to point(s) more or less adjacent the cathode 13, as compared with the anode l2, and also adjacent the tube wall It, such conductor(s) being electrically connected to a source of potential less negative or more positive than the cath-' ode. As shown in Fig. 1, the conductor means in question comprises a foraminous metal cylinder l5 of woven wire (e. a, 20 meshes per inch) that is substantially conformable to the interior of the envelope tube It and extends from adjacent the anode header 52 down to about the level of the cathode 3, but is separated from the periphery of the cathode; and this wire mesh lining I5 is connected directly to the anode header 12, as by one or more welds It within the cell. The effect of this conductor means l5 thus connected is to reduce very much the voltage V on the cell X required to produce any desired current value: e. at, while an ordinary cell without my invention may generally require a voltage of 7 volts to give only some 35 to 90 per cent of the saturation current, a cell with the wire mesh l5 may be expected to give 90 per cent of the saturation current on no more than 1.2 volts.

Fig. 2 shows a cell X equipped with an annular band or hoop l5a of thin sheet metal or foil extending around in the envelope [0 adjacent the vitreous wall, preferably in contact therewith, with a sheet metal strip 16a welded across the anode part l8 diametrally of the vitreous tube 10 and having its ends bent down adjacent the vitreous wall and connected directly'to the hoop l5a. As shown, the band I511 is at a level in the envelope l0 intermediate the end electrodes and sufficiently close to the cathode [3 to exert an effect very similar to that of the wire mesh l5: e. g., a tube so equipped may be expected to give 90 per cent of saturation current for a voltage of about 3 volts.

Fig; 3 shows a cell X equipped wlth wires i512 welded across the anode part 18 diametrally of the vitreoustube l0 and bent to extend down along the wall It parallel to its axis to points about level with the periphery of the cathode I3. As

here shown, the wires l5b are arranged at right angles so that four equally spaced straight wires extend along the wall I0. This arrangement gives about the same results as that shown in'Fig. 2.

In Figs. 2 and 3 various parts and features are marked with the same reference characters as their homologues in Fig, l, as a means of dispensing with repetitive description.

Fig. 4 shows comparative voltage-current curves A, B for photocells equipped as illustrated in Figs. 1 and 2 or 3, respectively, and curves C, D for photocells having about maximum and minimum amounts of caesium (invisible in both cases) coating the wall l0, but 'without features l5, [5a, [5b.

As to why the conductor arrangements above described improve the degree of current satura tion in the cell X in relation to the applied voltage, it appears that when the cathode l3 of a cell X without such a conductor arrangement is illuminated, some of the electrons photoelectrically emitted from the cathode collect on the inner surface of the glass wall l0 as a negative charge, whichacts (like the negatively charged grid of a triode) to oppose the passage of electrons from the cathode to the anode. In other words, the voltage required to produce a given space current from the cathode to the anode is materially higher than if this negative charge on the wall I!) were not there. conductor arrangements described isto-counteract or neutralize the opposition to the space current due to collection of ions on the wall Ill, to

a'greater or less degree, thus lowering the'volt age on the-cell X that is required to produce a space current of iven value. Indeed, the con ductor [5, a, or 15b results in actual neutralization ofthe negative wall charge to a very considerable extent, as will readily be understood bythose versed in electrical science.

It is further to be remarked that in the manufacture of photocells X by usual methods involving thermal deposition of caesium on the cathode I3 after completion of the cell structure, some caesium is also naturally and unavoidably deposited on the inner surface of the vitreous wall l0, although the layer is so thin as to be almost monatomic. It follows that light incident on the wall It! causes photoelectric emission of negative electrons from the wall, resulting in a reduction or partial neutralization of its negative charge previously mentioned. If the incident light is' well diffused over. the whole area of the wall Hi, the current reaches 90 percent saturation at a lower applied voltage than when only a ray or beam of light shines into the cell X on the cathode l3, as in the ordinary use of the cell.

, In Figs. 2 and 3, various parts and features .are'marked with the same reference characters as those corresponding in Fig. 1, in order to dispense with repetitive description, a distinctive letter being added where such distinction is desirable.

What I claim as new and desire to secure by Letters Patent of the United States is: a

1. A high vacuum space discharge 'phototube comprising a light-permeable vitreous tubef leave the light shining through the lateral tube The effect of the wall on said activated cathodesurface substantially unobstructed; said conductor serving to neutralize negative wall charge on the lateral tube wall, without substantial diminution of the illumination of the cathode surface. V

2. A high vacuum space discharge 'pho'totube comprising a light-permeable vitreous tube;

anode and cathode electrodes at opposite ends of said tube and adapted tocoactwith each other, said cathode being activated on its surface toward the anode and being exposed to the tube wall and thus photoelectrically responsive to light incident thereon through the lateral tube wall; and a conductor extending around inside the tube adjacent the lateral tube wall intermediate said electrodes and electrically connected directly to said anode, whereby it has substantially the anode potential, and being of small area compared to the tube wall area to leave the light shining through the lateral tube wall onsaid activated cathode surface substantially unobstructed; said conductor serving to neutralize negative wall charge on the lateral tube' wall, without substantial diminution of the illumination of the cathode surface.

3. A high vacuum space discharge phototube comprising a light-permeable vitreous; tube;

anode and cathode electrodes at Opposite ends of said tube and adapted to coact with each other. said cathode bein activated on its surface toward the anode and being exposed to the tube wall and thus photoelectrically responsive to.

obstructed; said conductors serving to neutralize negative wall charge on the lateral tube wall, without substantial diminution of the illumination of the cathode surface.

4. A high vacuum space discharge phototube comprising a light-permeable vitreous tube; anode and cathode electrodes at opposite ends of said tube and adapted to coact with each other, said cathode being activated on its surface toward the anode and bein exposed to the tube wall and thus photoelectrically responsive to light incident thereon through the lateral tube wall; and a lining of wire gauze extending around the tube wall having one edge attached directly to the anode and its other edge adjacent but not touching the cathode and being relatively coarse to leave the light shining through the lateral tube wall on said activated cathode surface substantially unobstructed; said lining serving to neutralize negative wall charge on the lateral tube wall, without substantial diminution of the illumination of the cathode surface.

5. A high vacuum space discharge phototube comprising a light-permeable vitreous tube, metallic headers sealed to and closing the ends of said tube, anode and cathode electrodes at opposite ends of the tube adapted to coact with each other and mounted on and electrically connected to respective ones of said headers, said cathode being activated on its surface toward the 6 anode and being exposed to the tube wall and thus photoelectrically responsive to light incident thereon through the lateral tube wall, and a lining of wire gauze extending around the tube wall having one edge attached directly to the header carrying the anode and its other edge adjacent but not touching the cathode, said gauze being relatively coarse to leave the light shining through the lateral tube wall on said activated cathode surface substantially unobstructed and said gauze serving to neutralize negative wall charge on the lateral tube wall, without substantial diminution of the illumination of the cathode surface.

A. HADLEY TAYLOR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

